Balloons mounted on the distal ends of catheters are widely used in medical treatment. The balloon may be used to widen a vessel into which the catheter is inserted or to force open a blocked vessel. The requirements for strength and size of the balloons vary widely depending on the balloon's intended use and the vessel size into which the catheter is inserted. Perhaps the most demanding applications for such balloons are in balloon angioplasty in which catheters are inserted for long distances into extremely small vessels and used to open stenoses of blood vessels by balloon inflation.
Applications such as balloon angioplasty require extremely thin walled high strength relatively inelastic balloons of predictable inflation properties. Thin walls are necessary because the balloon's wall and waist thicknesses limit the minimum diameter of the distal end of the catheter and therefore determine the limits on vessel size treatable by the method and the ease of passage of the catheter through the vascular system. High strength is necessary because the balloon is used to push open a stenosis and so the thin wall must not burst under the high internal pressures necessary to accomplish this task. The balloon must have some elasticity so that the inflated diameter can be controlled, so as to allow the surgeon to vary the balloon's diameter as required to treat individual lesions, but that elasticity must be relatively low so that the diameter is easily controllable. Small variations in pressure must not cause wide variation in diameter.
At the same time, the balloon must also possess excellent crossing, recrossing and tracking properties. It is highly desirable for the balloon to be expandable more than once to a reproducible expanded position from an initial folded position. It is also desirable that on deflation the deflated balloon return to its initial configuration. Finally, it is desirable for the balloon to be easily maneuvered through the body lumen. These properties are determined in large measure by the cone section of the balloon. Because the hoop stress is lower in the cone than in the body of the balloon, material comprising the cone section need not be a strong as the material in the body section. Thus, materials which are softer and more flexible may be used to form the cone section, thus allowing for greater crossing and recrossing properties while facilitating tracking.
There are a number of different approaches in the prior art to designing a balloon having differing physical and mechanical properties in different sections of the balloon.
One such approach involves forming a balloon by stretching and blowing of the balloon from a segment of extruded polymer tubing. Balloons produced by stretching and blowing a tubular preform or "parison" typically have much thicker waist and cone walls than the wall thickness of their body portions. The thicker cone walls contribute to the overall thickness of the catheter, making tracking, crossing and recrossing of lesions more difficult. Further, thick cones interfere with refolding of the balloon on deflation so that the deflated balloon can only be further inserted or withdrawn with difficulty, occasionally even damaging the blood vessel.
While there have been several solutions proposed for reducing the cone or waist thickness of catheter balloons in U.S. Pat. No. 4,906,241, U.S. Pat. No. 4,963,313, U.S. Pat. No. 5,304,340, U.S. Pat. No. 5,087,394, EP 318,919, EP 485,903, the procedures involved in these references are quite cumbersome.
Another approach involves the formation of layered balloons, wherein more than one layer of material is employed in certain regions of the balloon. The layers may be of a same or different material.
To that end, U.S. Pat. No. 5,358,486 to Saab discloses a multiple layer balloon. The balloon is built from a plurality of layers of material. The inner layer defines a complete layer. Each subsequent outer layer is trimmed to be shorter than the next adjacent innermost layer. The trimming is effected in the cone regions.
WO 95/09667 discloses a dilatation balloon formed of layers of a noncompliant structural polymer inner layer and a soft, abrasion resistant, elastomeric outer layer.
Unfortunately, the use of layering to achieve certain physical and/or mechanical properties can result in a thicker balloon than is desired.
It is a goal of the present invention to provide a balloon for a catheter device that is characterized by high strength and excellent crossing and recrossing properties while having a soft tip for ease of tracking, without the drawbacks of the prior art devices.
For the sake of clarity, the term `segment` shall refer to the individual pieces of material that are joined together to form the balloon. As such, the term `segment` is intended to include tubing and more generally, preforms. The term `section` shall refer herein to a region of a balloon or balloon preform (such as, for example, a cone section, a body section or a waist section).